Endoscopic Ultrasound With Fine-Needle Biopsy for Rare Pancreatic Metastasis: A Case Series of Neuroendocrine Transformation From Primary Prostate Cancer.

Metastatic cancer to the pancreas is uncommon, accounting for only 2%-4% of pancreatic malignancies. Prostate cancer spreading to the pancreas is exceptionally rare. We present 2 cases of metastatic castration-resistant prostate cancer with histologic transformation from adenocarcinoma to neuroendocrine prostate cancer. Both patients developed new pancreatic lesions, in which endoscopic ultrasound-guided fine-needle biopsy confirmed the diagnosis. These cases highlight therapy-emergent neuroendocrine prostate cancer with loss of prostate lineage markers (prostate-specific antigen, NKX3.1), positive neuroendocrine markers (synaptophysin, INSM1, chromogranin), very high proliferative indices, and serum marker discordance that can mimic primary pancreatic cancer. These scenarios present a diagnostic challenge with an aggressive and complex nature, requiring multidisciplinary management. With newer therapies and prolonged survival of patients with prostate cancer, clinicians will likely encounter more cases in the future. Gastroenterologists can play a key role in diagnosis and palliative treatment.